I haven't been to WDW with my family (first trip next month!), but every couple of years we all stay at DVC Hilton Head - my parents, siblings and their families. It is the best week of the year, and suprisingly enough we all get along very well on vacation. (we do pretty well the rest of the year, which helps too

) We just decide ahead of time that we are all on our own, and can do what is best for our family. If somebody joins us, great, but nobody has too. My parents usually go out at least one night on their own, my brother always has his own agenda, and we usually all end up together more than we are apart. But nobody feels guilty if they do something away from the rest. It's the only way to all get along.
My parents are meeting us at WDW for part of our week. This will be great, since they want to see the kid's reaction to everything, and having been there before, they can help us a lot. But they've made it clear that this is OUR vacation, and we get to make the plans. If they don't like them, they are free to do something else. Just remember it is your vacation, and you need to plan according to your family. I'm willing to bet your inlaws will agree to most things, and the memories your children will have sharing the magic with their grandparents will be the best.